1. Field of Invention
This invention relates to high strength-high density uranium alloys; and more particularly to ingot cast uranium-titanium-hafnium ternary metal alloys having enhanced mechanical properties compared with uranium-titanium binary metal alloys.
2. Brief Description of the Prior Art
The need for high density ballistic alloys of improved strength and ductility has long been recognized. Uranium, with a density of 19.05 g/cm.sup.3, is a well known candidate material for application in ballistic penetrator cores. Pure uranium, however, has a relatively low tensile strength (approximately 30 ksi). As a result, extensive efforts have been made to increase the tensile strength of uranium alloys while maintaining useful ductility. The results of these efforts culminated in the development of uranium-3/4Ti(wt %) alloy. Mechanical and ballistic properties of that alloy are described in the National Materials Advisory Board Report NMAB-350 (1980). This report, while recommending the use of U-3/4Ti for ballistic penetrator cores, also notes that improvement in mechanical properties must be made to address current and future counter threats in armor technology.
Typically, uranium-titanium metal alloys are cast into ingots and subsequently thermomechanically worked into plate or rod stock via techniques such as rolling or extrusion. As a final step, the alloys are given a high temperature anneal, typically at 800.degree. C., causing the room temperature (orthorhombic) crystal structure of uranium to transform into the high temperature .gamma. (bcc) crystal structure. This transformation results in solutionization of the titanium into the uranium lattice. The alloys are then strongly quenched (at quenching rates greater than 100.degree. C./sec) to room temperature, freezing the titanium into solution. Since titanium is not normally soluble in the room temperature alpha phase, a metastable martensitic variant, denoted .alpha..sub.a ' is formed to accommodate the supersaturated titanium.
The strengthening mechanisms in uranium-titanium alloys have been summarized by Eckelmeyer in "Diffusional Transformation, Strengthening Mechanisms, and Mechanical Properties of Uranium Alloys", from Metallurgical Technology of Uranium and Uranium Alloys (1981), page 129. The strength of uranium-titanium is attributable to several components. Primary strengthening arises from solid solution strengthening resulting from titanium supersaturation in the martensite. This supersaturation is also the basis for a precipitation hardening mechanism, by way of which aging at temperatures at or near 350.degree. C. causes formation of very fine precursors to the U.sub.2 Ti phase. As aging time continues, the volume fraction of precipitates increases, causing the strength to improve and the ductility to decrease. Ultimately, a peak in the hardness occurs beyond which both strength and ductility decrease.
It has been well documented that both strength and ductility of uranium-titanium alloys is strongly dependent on the titanium concentration. Indeed, Koger and Hemperly, Y-DA-6665, Union Carbide Corp., Oak Ridge, Tenn., (1976) have demonstrated a threefold drop in tensile elongation as the titanium content was increased from 0.7 to 0.8(wt) %. Thus, a practical limit to strengthening by increasing titanium concentration is reached due to a rapid decay of tensile elongation, a measure of ductility.